A room east of the starting area has a zombieman guarding two armor bonuses and a stimpack. This room has a misaligned wall on the west side. Open this wall to reach a secret area holding more stimpacks and bonuses. (sector 19)

On the northern side of the nukage pool at the start, jump into the nukage on the right side and into the southeast corner. This corner hides a lift leading up to a chainsaw. (sector 45)

Just before the exit door, open the misaligned wall to the right for two armor bonuses, a stimpack, shotgun shells, and an ammo clip. (sector 136)

The wall between the two silver bars in Secret #3 can be lowered as a lift. Follow the health bonuses and drop down into a room with armor bonuses, where the walls in the previous room open up revealing more bonuses as well as imps. (sector 137)

In Secret #1, approaching the southeast corner causes the lift to lower. If you try to leave Secret #1 through the way you came in, you will have to press a switch to open the way back. This switch not only opens the way back, but also opens a wall at the top of the lift. You need to time it so that the lift lowers, then you press the switch, then run back to the lift to reach the new opening. When you manage to do that, take the next lift down into the outside area where you can get a backpack (and a chaingun on easy difficulty). (sector 150)